The invention pertains to multi-layer polymeric formulations for protecting or insulating metallic objects and more particularly, to an integrated, tri-layer, thin-wall insulation composite, for use in a high temperature, automotive-wire article.
Applying polymeric formulations to the surface of metallic objects has been a long practiced art. Typically, this is performed for several different reasons. Firstly, metallic objects are prone to oxidation or other chemical attack of their surfaces, degrading the utility of the metal object. Secondly, metals are inherently electrically conductive, metallic articles used for this purpose must be insulated to maintain the integrity of the electrical circuit. Examples of applications of such articles include such diverse uses as building wire, aircraft wire, plenum wiring for local area network computers and communication, transformer wiring for isolating magnetic and electrical wiring, underground wiring, and electrical conductive wiring for automotive and general electrical machinery.
Useful materials for covering metallic objects for the above purposes are organic polymer formulations. However, because of the varied uses of these metallic articles and the conditions that they are subjected to, it is difficult to find an ideal organic polymeric formulation that will itself survive under such conditions and uses, yet still provide a multitude of critical properties. Common properties include such physical and chemical characteristics as durability, flexibility, elasticity, toughness, adhesion, thermal stability, chemical resistance, flame retardance and minimal smoke generation. In some cases the protective layer may even be required to have properties that seem almost to be antipodal (e.g., abrasion resistance and flexibility).
Of the plethora of organic polymer compositions that are known or theoretically available, derivatives of polyethylene, especially crosslinked polyethylene, have secured a significant share of the market for these applications. Polyethylene homo- and copolymers offer a wide range of beneficial properties especially with regard to flexibility and elasticity. The common non-halogenated polyethylenes are also inexpensive to produce and relatively easy to process. However, they typically have poor abrasion resistance and impact resistance. To a degree this can be improved by crosslinking, but a trade-off in other properties are then observed. Lastly, and in some cases most importantly, the unhalogenated polyethylene derivatives are not chemically inert and will degrade under a variety of conditions. This degradation is observed during aerial exposure and on contact to certain metals, in particular, copper. Degradation is accelerated when the polyethylene derivatives are stressed, for example under elevated temperatures or physical distortion. Halogenated polyethylenes, specifically fluorinated derivatives, have superior chemical inertness; however, they suffer from poor adhesive properties and are extremely costly.
There is still a long standing need to find a polymeric system that will provide all the properties required for a metal protectant or insulation especially in uses that are physically stressful, that require longlife, that involve human safety, and that are difficult to replace. A number of these criteria exists for high-temperature insulation for automotive wiring.
Automotive wire located under the hood in the engine compartment (engine wire) has traditionally been insulated with a single layer of high-temperature insulation that is disposed over an uncoated copper-wire core. The U.S. specification that is usually used for this wire is SAE J1128, rev. January 1995 type TXL. The high temperature requirements for type TXL insulation, which require specific tensile and elongation standards as well, specifies that the insulation be oven aged without the conductor at 155xc2x0 C. for 168 hours.
For certain newer automobiles, the high temperature specified for type TXL insulation is not sufficient to cover the actual temperatures existing in automobile engine compartments. The new requirements for engine wire range from 135xc2x0 C. to 180xc2x0 C. A typical test procedure now required is aging the insulation with the conductor for 3,000 hours at 150xc2x0 C. as specified in International Standard ISO 6722-1-and 6722-2, rev. January 1996.
Most of the high-temperature wire used as engine wire in North America uses crosslinked polyethylenes (XLPE) as the insulation material. European manufacturers have obtained good high-temperature performance (3,000 hrs. at 155xc2x0 C.) using thermoplastic polyesters. This insulation has outstanding resistance to gas and oil, is mechanically tough, and resists copper catalyzed degradation. Thermoplastic polyesters, however, can prematurely fail, because of hydrolysis. Thermoplastic polyester insulated wires have also been found to crack when exposed to hot salty water. They have also failed temperature humidity cycling as specified in the United States Car Specification PF-9600, Change A. As a result of these weaknesses, the use of thermoplastic polyester insulation has been limited in North America.
In addition to the foregoing discussion, the amount of wiring in automobiles has increased exponentially, as more electronics are being used in modern vehicles. This dramatic increase in wiring has motivated automobile manufacturers to reduce overall wire diameter by specifying thinner wall thicknesses, and specifying smaller conductor sizes. The forthcoming ISO 6722 specification (referred to as ultra-thin wire) reduces the insulation wall thickness to 0.20 mm. One automobile manufacturer in North America has released a new specification requiring a 0.15 mm wall thickness.
These reductions in insulation wall thicknesses pose manufacturing difficulties for wire fabricators. For XLPE, the thinner wall thickness of the insulation results in shorter thermal life, when aged at oven temperatures between 150xc2x0 C. and 180xc2x0 C. This limits their thermal rating. For example, a copper wire with an XLPE insulation having a 0.75 mm wall thickness is flexible, and does not crack when bent around a mandrel after being exposed to 150xc2x0 C. for 3,000 hours. But this same copper wire, with the same XLPE insulation having a 0.25 mm wall thickness, becomes brittle after being exposed to 1500xc2x0 C. for 3,000 hours.
The deleterious effects created by these extremely thin wall requirements have been attributed to copper catalyzed degradation, which is widely recognized as a problem in the industry.
XLPE insulation has many of the desired properties of the polyester materials, in addition to possessing good resistance to water. However, this material degrades when it comes into contact with copper at high temperatures.
It is possible to tin coat the copper core in order to prevent the copper from contacting the XLPE, but the additional cost of tin and the tin coating process are expensive. In addition, most automotive specifications require that the copper core be uncoated.
It is also possible to add copper stabilizers to the polyethylene compound, but copper stabilizers (such as Ciba Geigy Irganox MD-1024) yield only partial protection for wire having thin wall thicknesses, when used at 150xc2x0 C.
Polyolefins are also known to undergo oxidative degradation, especially at elevated temperatures. This defect has been long known to those in the art but the solution to the problem is not easy to obtain without significantly degrading other beneficial properties of the polyolefin.
Polyolefin insulated wires have been overcoated with a protective layer of fluorocarbon polymer (see U.S. Pat. No. 5,281,766 to Hildreth). This patent utilizes a fluorocarbon polymer as a protectant layer over the polyolefin insulation to protect against varnish. The varnish, which comprises a polyester or epoxy base, is applied during a bake process. If varnish is inadvertently applied to the polyolefin insulation, the insulation becomes brittle and cracks on bending. The fluorocarbon polymer disclosed in the ""767 patent is polyvinylidene fluoride or a polyvinylidene co-polymer having a maximum thickness of 5mil. However, no mention is made of the problem of oxidation of the polyolefin. Neither are any data presented to teach the increased useful life of the fluorocarbon polymer protection overcoat. Furthermore, no mention is made of the need for, or benefit of, multiple insulative underlayers.
Other prior art references disclose multilayer polymeric layers disposed on a metal wire or cable. For example, see U.S. Pat. Nos. 5,326,935 and 5,362,925 to Yamaguchi et al., where a three layer polymer insulation is described for wiring within a transformer. The examples that are given are for each of the three layers to be composed of a fluorinated polymer or other high-performance polymers such as polyurethane, polyimides, polyphenylene sulfide and the like. No mention is made of polyethylene. The problem that was solved in ""925 and ""935 reference related to thermal, not chemical, decomposition.
U.S. Pat. No. 5,462,803 , to Wessels, describes the use fluorocarbon polymers as an inner layer in a wire insulation. The outer layer is composed of polyvinyl chloride and the design is used to prepare fire-resistant plenum cable. Wessels states that fluoro-containing polymers, while being excellent insulators, are expensive. Thus, it is preferable that the thickness of the inner layer be kept to a minimum. No mention is made of unhalogenated polyolefins as is present in the current invention.
Other references relating to multilayered polymeric coating of metallic articles include U.S. Pat. No. 4,801,501 to Harlow, U.S. Pat. No. 4,988,835 to Shah, U.S. Pat. No. 5,059,483 to Lunk et al., U.S. Pat. No. 5,426,264 to Livingston et al., U.S. Pat. No. 4,521,485 to Tondre et al., U.S. Pat. No. 5,371,325 to Kalola et al., U.S. Pat. No. 5,841,072 to Gagnon, and U.S. Pat. No. 5,358,786 and U.S. Pat. No. 5,521,009 to Ishikawa et al.
However, none of the above references, either alone or in combination, teach or suggest either the composition of the present invention or a means of increasing the useful life of an olefinic polymer such as polyethylene in an insulative structure by simultaneously mitigating both the copper and aerial degradation pathways through the use of a ternary or tri-layer polymeric design. In virtually all cases these patents are directed toward solving problems regarding flammability, flexibility, or physical toughness. To solve such problems, the prior art patentees have looked towards combinations of polymeric components that are costly, overlooking the possibility of using less expensive materials such as polyethylene.
The present invention reflects the discovery that desired performance of polyolefinic insulation can be achieved by utilizing an integrated composite of insulation layers, rather than a single layer. The composite comprises a first, inner layer of a fluorocarbon polymer or other polymer having resistance to metal catalyzed degradation; a second, intermediary layer comprising polyolefin polymers; and a third, outer layer of a fluorinated polymer that is resistant to oxidation. The outer layer is designed to be chemically resistant to automotive fluids and salt water, as well as to high temperatures. The outer layer is also designed to provide the required mechanical protection. This composite insulation makes the wire suitable for under-the-hood automotive applications.
Other polymers that can be used as a first, inner layer to prevent copper migration into the XLPE include such high temperature polymers selected from a group consisting of: polyether sulfone, poly-ether-ketone, polyetherimide, and thermoplastic polyester. These polymers can be used in place of the first, inner fluorocarbon polymer. In another embodiment of the invention, optional adhesive layers are provided which can be incorporated into the insulative structure between any of the three polymer layers.
The insulation for metal articles of the present invention provides a beneficial combination of physical and electrical properties. The outer layer provides excellent resistance to physical abuse. The inner layer is preferably more flexible than the subsequently coated layers and thus provides insulation which can be physically tough yet flexible.
In accordance with the present invention, there is provided a high-temperature, automotive-wire article having a thin wall construction and an operative temperature that is greater than 135xc2x0 C. The wire article comprises an inner core of metal that is surrounded by a polymer insulation, the latter including an integral composite of three layers. The first, inner layer of the composite comprises a polymeric material that is insensitive to copper catalyzed degradation, while the second, intermediate layer of the composite comprises a polyolefin material that is preferably crosslinked by irradiation. Irradiation cross-linking requires a dose of approximately 60 to 400 kGy, and preferably between 100 and 200 kGy. The third, outer layer of the insulative tri-pack is a fluorinated polymer having excellent physical toughness and chemical inertness. It is specifically selected to limit the oxidation of the underlying polyolefin layer.
The XLPE intermediary layer can be chemically cross-linked or can be cross-linked by irradiation processing. The irradiation process of cross-linking the polyethylene suggests that certain fluorocarbon homopolymers made from monomers such tetrafluoroethylene and hexafluoropropylene should preferably not be used as the first, inner polymer unless irradiation is not required. This is because their properties are adversely affected by irradiation. However, fluorocarbon copolymers, such as ethylene-tetrafluoroethylene, can be irradiation cross-linked with a reactive monomer, such as triallyl isocyanurate, which increases the high temperature capability.
The inner, intermediary, and outer layers can either be bonded together with the use of an adhesive, or thermally fused together to form an integral insulation. Furthermore, this integral insulation can readily be removed from the conductor with commercially available equipment. The total wall thickness of the composite is generally less than 0.5 mm, and usually between approximately 0.15 mm. and 0.41 mm. When the first, inner layer is composed of a fluorocarbon polymer it is preferably made of a fluorocarbon polymer that contains CH2 groups as part of the polymer chain. Methylene groups within the backbone are necessary to create crosslinks during the optional irradiation process. The inner layer has a thickness in the range of between approximately 0.025 mm and 0.13 mm, and preferably less than 0.05 mm.